This invention relates to a switching arrangement and more particularly, but not exclusively, to an arrangement employing a plurality of solid state switches.
Previously, modulators used to drive rf devices, such as magnetrons, or for providing variable pulses for test equipment have used pulse transformers. More recently, we have previously proposed the use of a solid state switching arrangement in which a plurality of switch modules are connected together to give flexibility in operating voltage, duty cycle and pulse width with PRF""s up to 750 kHz.
The present invention arose from consideration of the implementation of a solid state switching arrangement but it is envisaged that it will be applicable to other types of switching arrangements also.
According to the invention, a switching arrangement for applying a pulse to a load comprises: switching means and annular capacitor means surrounding and coaxial with the switching means, the capacitor means being electrically connected to provide a current return path for current applied via the switching means to the load.
The annular distribution of the capacitor means around the switching means to give a coaxial assembly results in a substantial and desirable reduction of circuit inductance because of the coaxial current cancelling construction, with current travelling in one direction through the switching means and in the opposite direction through the capacitor means. The low inductance improves switching performance compared to another arrangement in which a capacitor is located physically remote from the switching means.
Preferably, the energy stored in the capacitance of the capacitor means is sufficiently large compared to the energy of the pulse output applied to the load that the capacitor means acts as an electrostatic screen. The charge of the capacitor means does not deplete significantly during an output pulse applied to the load and there is virtually no voltage change across its terminals. Thus, pulse displacement currents on the outside of the capacitor means are minimized and hence it is possible to provide electrostatic shielding in a particularly low volume by the annular configuration of the capacitor. This significantly reduces interference to other equipment located in the vicinity of the switching arrangement.
Preferably the capacitor means is contained in a housing having an outer wall and an inner wall, and which defines a tank in which the switching means is located. That is, the switching means is located within the volume bounded by the inner wall. A dielectric fluid may be contained within the tank, in the volume surrounded by the inner wall of the housing. The dielectric fluid may provide electrical insulation around the switching arrangement to prevent electrical breakdown. Alternatively, or in addition, the dielectric fluid may provide cooling for the switching means. In a preferred embodiment, the dielectric fluid is oil but other fluids may be suitable, for example, air cooling may be used.
The invention is particularly applicable where the switching means is solid state switching means. In one preferred embodiment, the solid state switching means includes a power FET. In alternative embodiment it could include an IGBT. The invention may be applied to a switching arrangement having a single solid state switch but is particularly advantageous where the switching means comprises a plurality of switches. These may be carried by a plurality of stacked modules with the switches being connected in series or in parallel. However, a single switch of any alternative technology may also be employed in the invention, such as a high power vacuum tube having a controllable input.
Where a plurality of stacked switching modules are used, it enables high voltage output pulses to be achieved, for example, of the order of tens of kilovolts, the number of modules may be of the order of 60 or more.
Preferably, the annular capacitor means comprises a plurality of capacitors distributed coaxially about the switch means. However, it could be embodied as a single annular capacitor in some arrangements. In one convenient arrangement, the annular capacitor means has a rectangular transverse cross-section, but other configurations could be used, for example, the annular capacitor could have a circular transverse cross-section.
Preferably, where the capacitor means comprises a plurality of capacitors, each capacitor comprises a plurality of capacitor elements connected in series. Thus, the capacitor elements are arranged along the longitudinal length of the switching means in a direction parallel to the current flow through the switching means. Preferably, the capacitor elements are arranged such that there is a nominally linear voltage gradient from one end of the capacitor means to the other. Thus, dc and low frequency electric field appear across the length of the device whilst pulse stress appears transversely to this across the smaller coaxial structure in a radial direction.
Where the capacitor means is fluid filled, then the housing of the capacitor means may communicate with the interior volume within which the switching means is located so that mixing occurs between the fluid in the capacitor volume and fluid in the switching means region where identical or compatible dielectric fluids are used. A fluid expansion system located separately from the capacitor means housing may be included to allow for expansion of fluid within the capacitor means.
In a preferred arrangement where the load includes a vacuum tube, for example, a magnetron, or some other device using a thermionic cathode, a heater supply is located at the pulse output end of the switching means.
An advantage of an arrangement in accordance with the invention is that impedance matching is not a design requirement or limitation.
A trigger circuit for the switching means may be included at the high voltage end of the switching means remote from the load to minimize displacement capacitance which occurs when the switching means is operated.